1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming process and a process cartridge, in which an alternating voltage is superimposed on a direct voltage and the voltage is applied to a charging member of roller-shape disposed proximity to a photoconductor, then the photoconductor is charged and an image is formed. More specifically, the present invention relates to an image forming apparatus, an image forming process and a process cartridge, in which the image quality and the durability of the photoconductor may be enhanced, images may be formed stably even after the repeated usages, and higher quality and higher stability of images may be established due to the superimposed alternating voltage as well as decrease of the charging hazard on the photoconductor.
2. Description of the Related Art
An image forming apparatus is typically constituted from a photoconductor (hereinafter, sometimes referring to “electrophotographic photoconductor” or “latent electrostatic image bearing unit”), charging unit, exposing unit (writing unit), developing unit, transferring unit, and fixing unit that fixes a toner transferred to a transferring paper, those units are usually disposed around the photoconductor. In addition, the image forming apparatus may be equipped with a cleaning member that cleans the toner remaining on the un-transferring photoconductor surface, and a charge-eliminating member to chancel the remaining charge at the exposing part of the photoconductor.
As for the charging member of such a image forming apparatus, charger of wire-like shape, charger of blade-like shape, charger of brush-like shape, charger of roller-like shape and so on may be exemplified. Among these, the chargers of corotron-type and scorotron-type are simple and convenient; in particular the chargers of corotron-type have been employed previously for a large number of apparatuses, since the charging is stable even at negative polarity that is the charging polarity of organic photoconductors. However, such problems have arisen that the generation of acidic gas (ozone gas, NOx, etc.) associated with the charging deteriorates the environment in use of the image forming apparatus, and degrades chemically the organic photoconductor mounted in the apparatus; therefore, the evacuation from the apparatus and/or the ventilation around the photoconductor is demanded, resulting in the gradual decrease of the market, except a part of higher speed apparatus or apparatus with larger diameter photoconductor. Further, the involvement of charger imposes to assure a space of a few millimeter between the photoconductor surface and the charger, and to utilize a power supply with larger capacity. Accordingly, the resultant apparatus is likely to become more expensive and larger size, these are factors to be avoided the employment of the charger.
By the way, in contact-charging members such as a charging blade, charging brush, charging roller and the like, wherein the photoconductor and charging member contact so as to charge the photoconductor, the generation of acidic gas is reduced and the capacity of the power supply is relatively small; therefore, the contact-charging members have been mainly employed for small size apparatuses. In particular, in the charging roller of roller-shape, the position may be adjusted by merely rotating in co-rotating direction or reverse-rotating direction in some case while contacting with the photoconductor, and the surface area is relatively wide compared to the charging blade and the like since the roller is utilized in rotating motion, therefore, the durability is relatively high. Accordingly, it is estimated that the majority of the charging members employed in the image forming apparatus are such charging rollers at present.
Such charging members of contacting type are disposed in a condition that the photoconductor and the charging member contact each other. Herein, the charging is caused essentially by a charging phenomenon from the charging member to the photoconductor. When the resistances of the charging member and the photoconductor surface are considerably low, the charging may be possibly introduced at the contacting area; however, the limitations in use are remarkable and the controlling is significantly difficult in general, since problems such as image flow are possibly induced due to the repeated usages when the resistance of photoconductor surface is low. Therefore, the resistance of the photoconductor is usually made larger such as at least 1010 Ω·cm or more such that the movement or diffusion of charge in horizontal direction is avoided at the photoconductor surface. In the image recording apparatus designed in such charging mechanism, the charging roller and the photoconductor do not contact each other, i.e. the contacting area does not exist, the charging phenomenon is induced at a significantly narrow gap of no more than 100 μm or so, consequently the photoconductor is subjected to charging.
Such charging at a narrow gap may be conducted referring to Bassen's discharging property in the usual range of photoconductor thickness. Therefore, the voltage at the photoconductor surface does not rise from zero volt even though a voltage is applied to the roller as applied bias, when the charging is only due to direct component (hereinafter, sometimes referring to “DC charging”); the photoconductor is subjected to charging when a voltage is applied at higher level than the discharge-initiating voltage. When the voltage is applied at higher than the discharge-initiating voltage, the surface voltage of the photoconductor exhibits a linear relation with the applied voltage and the slope of the relation is about one. However, in the case that the charging is conducted solely by direct component, the charging nonuniformity such as dot-like or band-like irregularity is often recognized when the developing is carried out in conventional way.
Japanese Patent Application Laid-Open (JP-A) No. 5-27556, for example, discloses that an alternating voltage of which the voltage between the peaks is more than two times of the discharge-initiating voltage in DC charging is superimposed duplicately, thereby the charging nonuniformity may be reduced.
The charging superimposed with the alternating voltage (hereinafter, sometimes referring to “AC charging”) may improve charging uniformity on the photoconductor compared to DC charging. However, such a problem exists that the abrasion wear of the photoconductor is significant in a practical condition, the durability of the photoconductor is decreased, and the confidence of the image forming apparatus is also decreased. As such, the charging uniformity on the photoconductor and the prolonged life of the photoconductor display a trade-off relation each other, it is significantly difficult to enhance the image quality produced by the image forming apparatus as well as to make more durable and stable the image forming apparatus.
On the other hand, the charging roller is utilized normally in contact with the photoconductor so as to make easy and simple to adjust the position of the charging rollers. When the remaining transferring toner exists on the photoconductor and occasionally the remaining toner exists after cleaning, these remaining toner is transferred to the surface of the charging roller thereby the roller is smeared, resulting in inferior charging. Further, when the toner smears the charging rollers, the surfaces of rollers may cause damages such as crazings, resulting in shortened life of charging rollers.
In light of these problems, a non-contact charging member is proposed, in which the surface of the photoconductor and the surface of the charging member are disposed in a close position. The non-contact charging member and the photoconductor are disposed in a separated condition such that a gap of 200 μm or less exists between the surface of the charging member and the surface of the photoconductor. JP-A No. 2001-188403, No. 2002-55508, No. 2002-148904, and No. 2002-148905 disclose the related technology.
In the non-contact charging member, it is necessary that an alternating voltage is superimposed on the direct voltage in order to stabilize the charging property on photoconductor or to make uniform the charging. The alternating voltage contribute to enhance the charging uniformity on the photoconductor compared to DC charging. However, such a problem also exists that the abrasion wear of the photoconductor is significant in a practical condition, the durability of the photoconductor is decreased, and the confidence of the image forming apparatus is also decreased. As such, the charging uniformity on the photoconductor and the prolonged life of the photoconductor display a trade-off relation each other, it is also significantly difficult to enhance the image quality produced by the image forming apparatus as well as to make more durable and stable the image forming apparatus.
In order to assure the charging uniformity of charging member when an AC charging is employed, such AC charging conditions as voltage between peaks (hereinafter, sometimes referring to “Vpp”), frequency, offset voltage of DC component (hereinafter, sometimes referring to “Vdc”) are controlled independently.
JP-A No. 5-27556, No. 2001-109235, and No. 2001-109238, for example, describe that Vpp is superimposed on the direct component at charging in a level of two times or more of charge-initiating voltage, thereby the charging of the photoconductor is stabilized.
JP-A No. 5-150564 and No. 2002-55512 describe that an alternating voltage is applied of which the frequency depends on the linear velocity of the photoconductor, thereby the charging is stabilized or uniformed.
As explained above, AC charging has been concerned to the charging uniformity on the photoconductor, and a charging hazard has hardly been considered with respect to AC charging.
JP-A No. 2002-214888 describes that the image deletion and the photoconductor wear may be decreased owing to controlling Vpp and controlling the charging current. However, the controlling is insufficient since the current measurements are difficult when the individual AC conditions are fluctuated such as the frequency is lower or the linear velocity is remarkably higher.
As explained above, the way to determine the charging conditions in order to assure the charging uniformity on the photoconductor and to decrease the charging hazard on the photoconductor, and also the achievement of image forming apparatus that produces images stably with high quality even after the repeated usages have been demanded.
On the other hand, JP-A No. 2001-33502 discloses an analysis of the charging condition between a charging member of roller-like shape and a photoconductor. In the analysis, the charging condition such as charging voltage on the photoconductor may be determined through a simulation of charging phenomena considering the shapes of charging member and photoconductor, physical values, and process conditions such as the applied bias and linear velocity of photoconductor, without the actual preparation of charging member or photoconductor or without carrying out the charging experiment.
However, the proposal was provided in order to manage the uniform charging on the photoconductor at the initial stage or virgin condition; i.e. the proposal does not concern to the degradation due to the repeated usage of the photoconductor in the image forming apparatus.
As explained above, such technology is still required that affords to simulate or decrease the charging hazard on the photoconductor even in the repeated usage of the photoconductor, and also that resolves the trade-off with the charging uniformity.
JP-A No. 2002-156877 discloses an image forming apparatus comprising a charging member that may charge a photoconductor through AC superimposed voltage, and a device that may coat an insulating material on the surface of photoconductor. The application concerns the problem that AC superimposed charging activates the surface of the photoconductor, the adhesive property of the photoconductor surface is raised, thereby the adhesion of toner additives is promoted, resulting in the shortening of the photoconductor life.
Although the application proposes the use of insulating material, when the insulating material is coated on the photoconductor and the charging hazard due to the charging member is lower, the performance of the photoconductor is deteriorated, resulting in adverse effects such as image blur. On the contrary, when the charging hazard is excessively large, the insulating material may degrade itself on the photoconductor, resulting in unstable coated quantity of insulating material. Further, the degraded insulating material may effect adversely on the photoconductor and the formed images, the control is not necessarily easy.
JP-A No. 2002-244516 discloses an image forming apparatus comprising a charging member that charges a photoconductor by AC superimposed voltage, and a unit for coating lubricant on the photoconductor surface. The proposal may prevent the filming or adhesion of toner on the photoconductor; however, the adverse effects of lubricant addition to the photoconductor surface and degradation of the lubricant may not be avoided, unless controlling the quantity of the charging hazard.
JP-A No. 2002-229241 discloses an image forming process in which a charging member applies charging on a photoconductor through AC superimposed voltage, lubricant is applied on the photoconductor surface, and the frequency of AC is 100 Hz to 2.5 kHz. The purpose of the application is also to remove adhered materials on the photoconductor surface essentially, and is not to decrease the charging hazard on the photoconductor. As explained later, specifying the AC frequency may not specify the charging hazard. In this proposal also, the adverse effects of lubricant addition to the photoconductor surface and degradation of the lubricant may not be avoided, unless controlling the quantity of the charging hazard.
JP-A No. 2002-156877, No. 2002-244516, and No. 2002-229241 disclose proposals in which cleaning ability of toner and toner additives is enhanced at the cleaning unit, thereby the toner and toner additives may be removed by the cleaning member. However, if the removal is possible, the abrasion wear of the photoconductor is higher; if the removal is not sufficient, the adverse effects are significant unless controlling the quantity of the charging hazard as explained above.
The charging uniformity on the photoconductor is one of the most important properties in charging the photoconductor by a charging roller. As for the factors with respect to the charging roller for the purpose of controlling the charging uniformity, the followings may be listed: the construction of charging roller such as layer thickness of the charging roller, the material of layer, and the like; roller diameter, bias applied to the roller such as Vpp, frequency, Vdc, and the like; gap between the charging roller and photoconductor, and the like. Further, as for the factors with respect to the photoconductor, the followings may be listed: process speed or linear speed of the photoconductor, photoconductor diameter, layer thickness, resistance of layer, dielectric constant of layer.
Thus, according to an orthodox approach, these factors are independently controlled, the charging roller and photoconductor are prepared experimentally and mounted to an actual image forming apparatus or experimental apparatus, then charging quantity on the photoconductor and surface potential are investigated according to necessities. Further, in order to investigate the adverse effect such as increased abrasion wear of the photoconductor as above explained, a running test including alternation with time is required in addition to the test of initial properties in terms of charging. Accordingly, time consuming and expensive laboratories are necessary in order to achieve the charging member or charging condition adapted to the image forming apparatus.